It is increasingly clear that T cell fate is the result of a multi-faceted and highly regulated process of differentiation. This process dictates multiple properties of the cell including the range of effector functions, survival, and functional avidity. Functional avidity is a fundamental determinant of in vivo efficacy. At present, we are far from a complete understanding of the molecular control of avidity. Our previous results demonstrated that an individual T cell can modulate avidity in response to the stimulatory signal. The overall goals of the studies presented here are to determine the mechanism by which CD8+ T cells modulate avidity, and the epigenetic changes associated with determination of the avidity set point. Aim 1. To define the regulation of IL-4 and IFN? production and signaling as a result of engagement of high versus low peptide. Our preliminary data support the hypothesis that avidity modulation in CD8+ T cells is regulated by the combined effects of T cell derived IL-4 and IFN?. We propose that encounter with high vs. low amounts of peptide/MHC results in divergence in cytokine production by T cells and/or their responsiveness to these cytokines. This model is innovative in that it suggests an autocrine cytokine based self tuning of avidity. Our studies will test the hypothesis that these individual cytokines are produced at distinct levels or with differet kinetics depending on the amount of peptide/MHC present. We will also test the hypothesis that cytokine receptor expression and/or responsiveness is differentially regulated in cells that are differentiating into high vs. low avidity effectors. Aim 2. To determine how cytokine and TCR signals induce epigenetic changes in cells undergoing functional avidity modulation. Epigenetic changes result in maintenance of gene expression programs through cell division, consistent with the stable functional avidity set point induced in effector cells in our system. Recent studie demonstrate the critical role of methylation in programming differentiation of CD8+ effector cells. We believe these results provide strong rationale for examining the role or methylation in establishing the avidity set point. These studies will provide novel information regarding the control of functional avidity in differentiating CD8+ T cells. The results from these studies may ultimately open the door to new therapeutic interventions aimed at manipulating T cell responses in vivo.